1. Field of the Invention
This invention relates to an air-fuel ratio control system for internal combustion engines, and more particularly to an air-fuel ratio control system of this kind, which carries out feedback control of the air-fuel ratio of a mixture supplied to the engine, in response to an output from an oxygen concentration sensor having output characteristics substantially proportional to oxygen concentration in exhaust gases from the engine.
2. Prior Art
A limiting current type oxygen concentration sensor generally employed in air-fuel ratio control systems for internal combustion engines has an output characteristic that an output current value thereof is proportional to partial pressure of oxygen in exhaust gases from the engine when a positive voltage is applied to the sensor. There has been already known an oxygen concentration detecting system which utilizes such output characteristic of the limiting current type oxygen concentration sensor to linearly detect the oxygen concentration in exhaust gases from an internal combustion engine (Japanese Laid-Open Patent Publication (Kokai) No. 59-163556). Limiting current characteristics of an oxygen concentration sensor of this type change with the temperature of the sensor, and therefore the temperature of the sensor must always be controlled within a predetermined activation temperature range so as to maintain required detection accuracy. The known oxygen concentration detecting system, therefore, detects the internal resistance of the oxygen concentration sensor assumed when a predetermined negative voltage is applied to the sensor and (controls heating of the sensor by means of a heater so that the detected internal resistance value is held constant to thereby maintain the sensor in an activated state, utilizing the nature of the oxygen concentration sensor that the output current from the sensor assume, a value proportional to the temperature of the sensor independently of the oxygen partial pressure when a negative voltage is applied to the sensor.
An air-fuel ratio control method for internal combustion engines using an oxygen concentration sensor of this type has been disclosed by Japanese Laid-Open Patent Publication (Kokai) No. 4-203233, which includes calculating an amount of fuel to be supplied to the engine using an air-fuel ratio correction coefficient set based on an output from the oxygen concentration sensor and a desired air-fuel ratio to feedback-control the air-fuel ratio of a mixture supplied to the engine to the desired air-fuel ratio, and setting upper and lower limits of the air-fuel ratio correction coefficient according to the desired air-fuel ratio so as to prevent excessive correction of the air-fuel ratio, thereby feedback-controlling the air-fuel ratio within a proper range.
On the other hand, with recent stricter regulation of exhaust gases, there is an increasing demand for starting the feedback control of the air-fuel ratio as early as possible after the start of the engine, and hence it is desired that the oxygen concentration sensor should become activated as early as possible after the start of the engine. Conventionally, to promote activation of the sensor, the sensor is heated by means of a heater. However, the sensor cannot be heated to the activation temperature instantly after the start of heating. Thus, immediately after the start of the engine, the oxygen concentration sensor is not fully activated, and therefore, until the sensor becomes fully activated, exhaust gases from the engine contain considerable amounts of unburnt HC and sulfur components and hence are in an unstable or unpurified condition.
If the air-fuel ratio feedback control is started at early timing after the start of the engine, using the above-mentioned conventional air-fuel ratio method which sets upper and lower limits of the air-fuel ratio correction coefficient irrespective of the activation state of the oxygen concentration sensor, the output from the oxygen concentration sensor can deviate to a richer side from a proper value, leading to inaccurate detection of the oxygen concentration. Consequently, an inappropriate amount of fuel can be supplied to the engine, which causes hesitation of the engine operation and degraded exhaust emission characteristics.